Page 53 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

PART I Basic Principles: How Traits Are Transmitted
3

chapter

Extensions to

Mendel’s Laws

In this array of green, brown, and red lentils, some of the seeds
have speckled patterns, while others are clear.
© PhotoLink/Getty Images RF

chapter outline

• 3.1 Extensions to Mendel for Single-Gene
Inheritance
• 3.2 Extensions to Mendel for Two-Gene Inheritance
• 3.3 Extensions to Mendel for Multifactorial
UNLIKE THE PEA traits that Mendel examined, most Inheritance
human characteristics do not fall neatly into just two
opposing phenotypic categories. These complex traits,
such as skin and hair color, height, athletic ability, and
many others, seem to defy Mendelian analysis. The same can be said of traits
expressed by many of the world’s food crops: Their size, shape, succulence, and
nutrient content vary over a wide range of values.
Lentils (Lens culinaris) provide a graphic illustration of this variation. A type of
legume, lentils are grown in many parts of the world as a rich source of both protein
and carbohydrate. The mature plants set fruit in the form of diminutive pods that con-
tain two small seeds. These seeds can be ground into meal or used in soups, salads,
and stews. Lentils come in an intriguing array of colors and patterns (Fig. 3.1), and
commercial growers always seek to produce combinations to suit the cuisines of dif-
ferent cultures. But crosses between pure-breeding lines of lentils result in some star-
tling surprises. A cross between pure-breeding tan and pure-breeding gray parents, for
example, yields an all-brown F 1 generation. When these hybrids self-pollinate, the F 2
plants produce not only tan, gray, and brown lentils, but also green.
Beginning in the first decade of the twentieth century, geneticists subjected many
kinds of plants and animals to controlled breeding tests, using Mendel’s 3:1 pheno-
typic ratio as a guideline. If the traits under analysis behaved as predicted by Mendel’s
laws, then they were assumed to be determined by a single gene with alternative dom-
inant and recessive alleles. Many traits, however, did not behave in this way. For some,
no definitive dominance and recessiveness could be observed, or more than two alleles
could be found in a particular cross (Fig. 3.1). Other traits turned out to be determined
by two genes. Yet other traits were multifactorial, that is, determined by several dif-
ferent genes, or by the interaction of genes with the environment. The seed color of
lentils is a multifactorial trait because color is controlled by multiple genes.
Because traits can arise from an intricate network of interactions, they do not
always generate straightforward Mendelian phenotypic ratios. Nonetheless, simple
extensions of Mendel’s hypotheses can clarify the relationship between genotype and

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